RE: [SLE] Difference in "ps -o pcpu" and top CPU percentage

From: Greg Wallace (jgregw_at_acsalaska.net)
Date: 08/25/05

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    To: <suse-linux-e@suse.com>
    Date: Wed, 24 Aug 2005 22:19:11 -0800
    
    

    On Wednesday, August 24, 2005 @ 8:57 PM, Richard Mixon wrote:

    >Greg Wallace <mailto:jgregw@acsalaska.net> scribbled on Wednesday,
    >August 24, 2005 9:23 PM:

    >> On Wednesday, August 24, 2005 @ 6:16 AM, Richard Mixon wrote:
    >>
    >>> Jerry Feldman <mailto:gaf@blu.org> scribbled on Wednesday, August 24,
    >>> 2005 5:55 AM:
    >>
    >>>> On Wednesday 24 August 2005 1:38 am, Richard Mixon (qwest) wrote:
    >>>>> We noticed a significant different between the "percentage CPU"
    >>>>> reported by TOP for a process (a Java instance of Tomcat) and that
    >>>>> reported by issuing "ps -o pid,pcpu <process-id>". TOP indicated
    >>>>> that the process was using 99% CPU, but "ps" just showed it using
    >>>>> 40%.
    >>>>>
    >>>>> I have google and googled, read the man/info pages - but cannot
    >>>>> find any explanation for how they each arrive at their figures.
    >>>>>
    >>>>> Any ideas or explanations are appreciated/welcome.
    >>>> the top(1) command uses a sample and measures the percentage of
    >>>> elapsed CPU time between samples. PS is a single sample where
    >>>> percentage of CPU time is expressed in a ratio between "CPU time
    >>>> used divided by the time the process has been running".
    >>
    >>> Jerry, thanks - this sounds good. But if top is time averaging cpu
    >>> usage, I would expect it to report a lower percentage cpu usage than
    >>> ps does.We are seeing the opposite. BTW, this is a dual Xeon with
    >>> hyperthreading turned on.
    >>
    >>> Any other ideas?
    >>
    >>> Thanks - Richard
    >>
    >> ps could be lower if that process had just started. I. e., it could
    >> be a heavy load now, but not as related to your total usage for the
    >> session (right?).

    >Greg, good idea but unfortunately the process had been at 99% CPU usage
    >in top for a number of minutes, but using ps it only showed 40%.

    > - Richard

    So the ps % wasn't slowly creeping up over time?

    Greg Wallace

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